1. Technical Field
This invention relates generally to apparatus for generating neutrons and in particular to neutron generators for subsurface applications.
2. Description of Related Art
The characteristics of geological formations are of significant interest in the exploration for, production and monitoring of subsurface water, oil and gas. To that end, a variety of techniques have been developed to measure subsurface characteristics and evaluate the obtained data to determine the petrophysical properties of interest. These techniques typically involve the subsurface deployment of tools or instruments equipped with sources adapted to emit energy into the formations (usually through a borehole traversing the formations). The emitted energy interacts with the surrounding formations to produce signals that are detected and measured by one or more sensors or detectors on the instrument. By processing the detected signal data, a profile or log of the subsurface properties is obtained.
A variety of logging techniques have been developed to evaluate subsurface formations. A number of such techniques involve emitting neutrons into the formation and evaluating the results of neutron interactions with formation nuclei. Neutrons have no electric charge and their mass is similar to that of a proton. The lack of charge allows neutrons to penetrate into formations. This property of neutrons makes it ideal for subsurface logging applications. In the formation, neutrons interact with matter in a wide variety of ways. The characteristics of some of these interactions can be used to measure the formation properties.
Various types of radiation sources have been used in subsurface logging systems. For example, neutrons or gamma rays may be generated simply through the use of radioactive isotopes (which naturally decay over time), an x-ray source may be used or neutrons may be generated in an electronic device utilizing a nuclear reaction generating neutrons on demand. U.S. Pat. Nos. 3,255,353, 4,810,459, 4,879,463 and 4,904,865 describe logging instruments equipped with active radiation sources and appropriate sensors. For neutron logging, the chemical source has the advantage of being virtually indestructible. It has no electronic parts, so it can be relied upon to always produce neutrons (zero downtime). However, this is also a disadvantage of the chemical source. Because the emission of neutrons cannot be shut off, strict radioactive safety procedures must be followed when handling the source and the instrument containing the source. This disadvantage prompted the development of electronic neutron sources.
High-energy neutrons may be generated through the controlled collision of energized particles by using a nuclear fusion reaction. Such a system is commonly referred to as a neutron generator. The generation of neutrons on demand by the use of energetic particle beams allows the construction of a neutron source which emits neutrons in bursts of well-determined duration and time sequences. One such pulsed neutron generator is described in U.S. Pat. No. 5,293,410. The neutron generator described in the '410 patent uses an accelerator tube in which charged particles, such as Deuterium ions, are accelerated through an electric-static potential and collide with a target element such as Tritium. The reaction of the Deuterium ions with the Tritium target produces almost monoenergetic neutrons at an energy level of about 14 MeV. In most applications the neutrons are not emitted continuously but in short bursts of well-defined durations and in repetitive sequences. When using such a pulsed neutron generator, the formation surrounding the instrument is subjected to repeated, discrete “bursts” of neutrons. U.S. Pat. Nos. 4,501,964, 4,883,956, 4,926,044, 4,937,446, 4,972,082, 5,434,408, 5,105,080, 5,235,185, 5,539,225, 5,219,518 and 5,608,215 describe logging instruments equipped with neutron generators.
FIG. 1 shows a “hot cathode” electronic neutron generator 10. These generators 10 usually have three major features:
(i) a gas source to supply the reacting substances, such as Deuterium (H2) and Tritium (H3);
(ii) an ion source comprising usually at least one anode and a cathode to emit electrons; and
(iii) an accelerating gap to impel produced ions to a target to generate nuclear reactions with energy expressed in millions of electron volts (MeV).
The neutron generator 10 of FIG. 1 uses a gas source 12 formed from a helically wound filament 14 coated with Zirconium, which when heated releases the gas. Under typical operating conditions, the filament 14 is heated by electric current to trigger the gas release, a cathode 16 is heated by electric current, and the emitted electrons are accelerated through an electric field to create an ion beam to strike a target 18 and generate neutrons. This conventional generator design requires a fair amount of electrical energy to power its components.
Many potential neutron generator applications require operation with a battery pack as a power source. Additionally, space applications of neutron generators for elemental surveys of planets and asteroids need systems that operate on as little power as possible. Conventional neutron generators have rather high power requirements. A need remains for improved neutron generators that require less power to operate.